'If there is no struggle, there is no progress," said the African American abolitionist Frederick Douglass. "Those who profess to favour freedom and yet depreciate agitation are men who want crops without ploughing up the ground; they want rain without thunder and lightning. They want the ocean without the awful roar of its many waters ... Power concedes nothing without a demand. It never did and it never will."

By the end of last week it looked as though the fortnight of struggle between minority French youth and the police might actually have yielded some progress. Condemning the rioters is easy. They shot at the police, killed an innocent man, trashed businesses, rammed a car into a retirement home, and torched countless cars (given that 400 cars are burned on an average New Year's Eve in France, this was not quite as remarkable as some made out).

But shield your ears from the awful roaring waters for a moment and take a look at the ocean. Those who wondered what French youth had to gain by taking to the streets should ask what they had to lose. Unemployed, socially excluded, harassed by the police and condemned to poor housing, they live on estates that are essentially open prisons. Statistically invisible (it is against the law and republican principle to collect data based on race or ethnicity) and politically unrepresented (mainland France does not have a single non-white MP), their aim has been simply to get their plight acknowledged. And they succeeded.

Even as the French politicians talked tough, the state was suing for peace with the offer of greater social justice. The government unrolled a package of measures that would give career guidance and work placements to all unemployed people under 25 in some of the poorest suburbs; there would be tax breaks for companies who set up on sink estates; a €1,000 (£675) lump sum for jobless people who returned to work as well as €150 a month for a year; 5,000 extra teachers and educational assistants; 10,000 scholarships to encourage academic achievers to stay at school; and 10 boarding schools for those who want to leave their estates to study.

"We need to respond strongly and quickly to the undeniable problems facing many inhabitants of the deprived neighbourhoods," said President Chirac. From the man who once said that immigrants had breached the "threshold of tolerance" and were sending French workers "mad" with their "noise and smell" this was progress indeed.

"The impossible becomes probable through struggle," said the African American academic Manning Marable. "And the probable becomes reality."

And the reality is that none of this would have happened without riots. There was no petition these young people could have signed, no peaceful march they could have held, no letter they could have written to their MPs that would have produced these results.

Amid the charred chassis and broken glass there is a vital point of principle to salvage: in certain conditions rioting is not just justified but may also be necessary, and effective. From the poll tax demonstrations to Soweto, history is littered with such cases; what were the French and American revolutions but riots endowed by Enlightenment principles and then blessed by history?

When all non-violent, democratic means of achieving a just end are unavailable, redundant or exhausted, rioting is justifiable. When state agencies charged with protecting communities fail to do so or actually attack them, it may be necessary in self-defence.

After the 1967 riots in American cities, President Johnson set up the Kerner commission. It concluded: "What white Americans have never fully understood - but what the Negro can never forget - is that white society is deeply implicated in the ghetto. White institutions created it, white institutions maintain it, and white society condones it." How else was such a damning indictment of racial discrimination in the US ever going to land on the president's desk?

Following the inner-city riots across Britain in 1981, Lord Scarman argued that "urgent action" was needed to prevent racial disadvantage becoming an "endemic, ineradicable disease threatening the very survival of our society". His conclusions weren't perfect. But the kernel of a message black Britons had been trying to hammer home for decades suddenly took centre stage. A few years later Michael Heseltine wrote a report into the disturbances in Toxteth entitled It Takes a Riot.

Rioting should be neither celebrated nor fetishised, because ultimately it is a sign not of strength but weakness. Like a strike, it is often the last and most desperate weapon available to those with the least power. Rioting is a class act. Wealthy people don't do it because either they have the levers of democracy at their disposal, or they can rely on the state or private security firms to do their violent work for them, if need be.

The issue of when and how rioting is effective is more problematic. Riots raise awareness of a situation, but they cannot solve it. For that you need democratic engagement and meaningful negotiation. Most powerful when they stem from a movement, all too often riots are instead the spontaneous, leaderless expression of pent-up frustration void of an agenda or clear demands. Many of these French youths may have had a ball last week, but what they really need is a party - a political organisation that will articulate their aspirations.

If Kerner and Scarman are anything to go by, the rioters will not be invited to help write the documents that could shape racial discourse for a generation. Nor are they likely to be the primary beneficiaries.

"During the 80s, everyone was desperate to have a black face in their organisation to show the race relations industry that they were allowing black people to get on," says the editor of Race & Class, Ambalavaner Sivanandan. "So the people who made this mobility possible were those who took to the streets. But they did not benefit." The same is true of the black American working class that produced Kerner.

Given these uncertain outcomes, riots carry great risk. The border between political violence and criminality becomes blurred, and legitimate protest risks degrading into impotent displays of hypermasculinity. Violence at that point becomes not the means to even a vague aspiration but the end in itself, and half the story gets missed. We heard little from young minority French women last week, even though they have been the primary target of the state's secular dogma over the hijab.

Finally, violence polarises. The big winner of the last two weeks may yet prove to be Sarkozy. The presidential-hopeful courted the far-right with his calculated criticisms of the rioters; if he wins he could reverse any gains that may arise. Le Pen also lurks in the wings.

The riots in France run all these risks and yet have still managed to yield a precarious kind of progress. They demand our qualified and critical support.

Power has made its concessions. But how many, for how long and to whom depends on whether those who made the demands take their struggle from the margins to the mainstream: from the street to the corridors of power.